During idle operation of a vehicle, loads may be intermittently and quickly applied to the vehicle's power system and draw from power supplied by the internal combustion engine. Loads include various electrical loads, mechanical loads, and those caused by environmental conditions. For example, loads are applied when the vehicle accelerates from idle (such when the vehicle accelerates from being stopped or from a constant speed), when an air conditioning compressor coupled to the engine is enabled or when an alternator responds to an increase in electrical power usage. During idle operation, the engine alone needs a relatively constant specific amount of energy to run the engine (e.g., to compensate for heat loss and frictional loss) and maintain a constant idle speed. However, to compensate for intermittent loads applied to the engine while maintaining constant idle speed operation, different approaches have been developed.
One approach is to control airflow into the engine with the throttle. However, the throttle provides a torque response that is often too slow to meet the demands of intermittently applied loads. Another approach is to control spark timing. Spark timing is measured with respect to crankshaft rotation for a spark ignited internal combustion engine, with Top Dead Center (TDC) of the piston compression stoke being considered 0 degrees. Advancing spark refers to igniting the spark device earlier in the piston rotation cycle. Retarding spark refers to igniting the spark later in the rotation cycle.
Ideally the spark timing commanded during idle operation is the spark advance that provides maximum brake torque timing (referred to as MBT timing). This provides the highest energy output of the engine for the given operating conditions and the amount of fuel used. Retarding spark timing from MBT timing reduces the power output from the engine. This has the effect of reducing the efficiency of the engine and requires more airflow and fuel to provide a given torque; but, it also has the effect of creating a torque reserve that can be used to meet the demands of quickly increasing loads on the engine via spark control by advancing spark on the next combustion cycle. However, as provided, retarding spark timing results in increased fuel consumption and a concomitant reduction in fuel economy.
In vehicles such as hybrid vehicles, one or more electric machine may work in conjunction with the internal combustion engine and may be used to respond to intermittent load demands. The electric machine can respond much more quickly to torque commands than throttle generated torque commands for the internal combustion engine. Consequently, the electric machine represents a torque actuator that may be used in place of spark retard generated torque reserve. Additionally, when a load is removed from the engine and less torque is required, the electric machine can quickly provide negative torque that can be used to charge an electrical energy storage device. This operation removes the necessity to even further retard spark for decreasing torque requests.
The electric machine may be powered by batteries or other electric energy sources (such as supercaps or fuel cells) and also has the ability to generate and store electrical energy. When the electric machine draws electrical energy, it provides positive torque, when the electrical machine generates and stores electrical energy, it provides negative torque.